1. Field of the Invention
The present invention relates to two-axis measurement systems, and more particularly, to a robotic scanner for performing planar near-field antenna measurements capable of operating in either a vertical or a horizontal orientation.
2. Description of Related Art
High performance antennas are increasingly prevalent in the art as spacecraft, aircraft, ship and ground vehicle mission requirements become more sophisticated. One problem in the development and manufacture of high performance antennas is the measurement of antenna performance. Traditionally, antenna measurement was conducted by placing the antenna at a remote location, and measuring the amplitude and phase response characteristics of the antenna in its operational range. Typical operational distances for high gain antennas can range from fifty feet to three miles. This measurement technique, known as far-field testing, suffers from significant limitations, such as susceptibility to weather effects, ground reflections and increasing real estate costs.
As an alternative to far-field testing, near-field testing was developed. A near-field test is conducted in an indoor test range using a microwave probe to sample the field radiated near the antenna under test (AUT). A computer collects the amplitude and phase data sampled by the microwave probe, and calculates the far-field equivalent response using a Fourier transform technique. Accurate near-field measurements require that all the significant antenna energy be sampled by the microwave probe. Highly directive antennas, such as reflectors and waveguide phased arrays, beam most of the energy in the forward direction normal to the antenna aperture. To test these types of antennas, a planar near-field robotic scanner is utilized to move the microwave probe along a planar pattern approximately normal to the antenna aperture. To accurately reconstruct the measured field, the probe must sample the antenna energy at a plurality of points with a minimum spacing based on the Nyquist sampling theorem. Near-field measurement systems of this nature are described in U.S. Pat. Nos. 5,408,318 and 5,419,631, both to Slater, and assigned to the same assignee as the present invention.
The physical configuration of the near-field robotic scanner will often depend on the size and performance characteristics of the antenna under test. An antenna having a particularly large aperture, or one that is gravity sensitive, may be measured using a horizontally oriented scanner. Such a scanner may be supported by a frame structure that envelopes the antenna under test and defines a horizontal plane over the antenna. The antenna would radiate directly upward and the probe would sample the antenna's energy as the probe moves through the horizontal plane. The frame may be rigidly and/or permanently attached to the ground so as to provide a highly stable platform. Such stability is necessary to provide a high degree of planarity and avoid undesired variations in the probe position that would reduce the accuracy of the measurement.
Conversely, smaller antennas may be measured using a vertically oriented scanner. These scanners may be lighter in weight and construction than the horizontally oriented scanners, thus providing a degree of portability for such measurement systems. The vertically oriented scanners also have a frame structure that, unlike the horizontally oriented scanners, defines a vertical plane alongside an antenna under test. This vertical orientation may be desirable for testing a spacecraft antenna having heat pipes which are gravitationally sensitive. The antenna would be oriented to radiate sideways with the antenna aperture disposed normal to the vertical plane and the probe sampling the antenna's energy as the probe moves through the vertical plane.
Given the distinct advantages of each of the horizontally and vertically oriented scanners, it would be additionally advantageous to provide a single robotic scanner capable of both horizontal and vertical operation. Such a system would yield substantial additional flexibility and resulting cost savings by permitting the same basic measurement system to be used to test a variety of different antenna sizes, types and orientations. Thus, a critical need exists to provide a robotic scanner for planar near-field antenna measurements which is capable of selectively performing in either a vertical or a horizontal orientation.